Close up of granny flat or annexe in garden

Annexes, also known as granny flats or self-contained living spaces, have gained popularity in recent years as a solution for multigenerational living or providing independent accommodation for elderly family members. However, one of the primary concerns when considering a granny flat or annexe is the cost. There are various factors that influence the cost of a granny annexe, including whether to convert existing space or build a purpose-built structure, and understanding them pre-project can go a long way in helping you make a solid project plan.

Granny annexes generally require a modest sized living space with kitchen area, bedroom and en-suite bathroom, and as such, 37 metres squared is a target size for a studio and a good basis for the costing.

Converting Existing Space for a Granny Annexe

Converting existing space within the main house or an outbuilding is often a cost-effective option. Often using a garage conversion or a barn conversion, on average, the cost of converting existing space is approximately £1,000 per square meter. This figure takes into account the necessary expenses for fully fitting and decorating the annexe. For an average project, the total cost could be around £37,000 + VAT.

One of the advantages of converting existing space is that some essential infrastructure, such as plumbing and electrical connections, may already be in place, reducing the overall cost. However, it’s important to note that the availability of suitable space within the existing structure can be a limiting factor, but still, using what is there and building in addition to it can reduce costs.

Purpose-Built Granny Annexe

Building a purpose-built granny annexe provides greater flexibility in terms of design and customisation, modernisation and possibly value too. While it is a more expensive option, it offers the advantage of a self-contained living space that is separate from the main house. On average, the cost for a purpose-built granny annexe is approximately £2,200 per square meter, including full fit-out and decoration. For an average project, the total cost could be around £81,500 + VAT.

The higher cost of a purpose-built granny annexe is due to various factors. It involves constructing a new building from scratch, including the foundation, walls, roofing, and all the necessary installations. Additionally, cost of materials, labour, and obtaining planning permissions can contribute to the overall expense.

Factors Affecting Costs

Several factors can influence the cost of a granny annexe beyond the basic conversion or construction expenses:

Size and Layout

The size and layout of the granny annexe play a significant role in determining the cost. A studio-sized annexe, measuring approximately 37 square meters, is a popular choice. However, larger or more complex designs will naturally incur higher costs.


The location of the granny annexe can affect the overall cost. Factors such as land prices, accessibility, and local construction costs can vary from one region to another.

Materials and Finishes

The choice of materials and finishes can impact the cost significantly. High-quality materials and luxurious finishes will increase the overall expense, while more budget-friendly options can help keep costs down.

Additional Care and Adaptations

If the granny annexe requires extra care features or disability adaptations, such as ramps, handrails, or wider doorways, these additional requirements will add to the overall cost. It is crucial to consider any specific needs to ensure the annexe is safe and accessible.


When considering a granny annexe, it’s essential to understand the factors that influence its cost. Converting existing space within the main house or an outbuilding can provide a more affordable option, with an average cost of around £37,000 + VAT for a fully fitted and decorated annexe. On the other hand, a purpose-built granny annexe offers greater design flexibility and privacy, but comes with a higher price tag of around £81,500 + VAT for an average project.

Remember, these figures are estimates, and costs can vary depending on various factors such as location, size, materials, and additional care requirements. To get a more accurate understanding of the costs involved, it is recommended to consult professionals and obtain detailed quotes tailored to your specific needs.

Ultimately, the cost of a granny annexe is an investment in providing a comfortable and independent living space for your loved ones. With careful planning and consideration, you can find a solution that suits your requirements and budget, ensuring a harmonious multigenerational living arrangement for years to come.